The present invention relates to an eddy current testing probe which is usable in nondestructive testing.
Eddy current testing probes have been developed for use in nondestructive testing, which are performed, for example, during manufacturing of steel or nonferrous products or maintenance of heat exchanger tubes in various plants. Basically, testing of a specimen by using an eddy current testing probe is performed by generating an eddy current at a surface of the specimen by an excitation coil, and monitoring change in impedance of a detection coil due to an influence of the eddy current to detect a flaw. When a flaw exists at the surface of the specimen, the flaw affects the eddy current generated at the surface of the specimen. When the eddy current changes, an influence of the change in the eddy current appears in the impedance of the detection coil. Therefore, the flaw in the specimen can be detected by monitoring the change in the impedance of the detection coil.
FIGS. 7(a) to 7(d) are schematic diagrams illustrating the constructions of various types of conventional eddy current testing probes, where the eddy current testing probes can be sorted into the respective types based on their modes of detection of the change in the impedance and their constructions of the excitation coils and the detection coils.
Based on the modes of detection of the change in the impedance, the eddy current testing probes can be sorted into an absolute type and a differential type. The eddy current testing probe of the absolute type detects the flaw in the specimen by a detection coil, as illustrated in FIGS. 7(a) and 7(b), and the eddy current testing probe of the differential type detects the flaw in the specimen based on a difference between amounts of impedance generated in a pair of detection coils, as illustrated in FIGS. 7(c) and 7(d).
Based on the constructions of the excitation coils and the detection coils, the eddy current testing probes can be sorted into a selfinduction type as illustrated in FIGS. 7(a) and 7(c), and a mutual-induction type as illustrated in FIGS. 7(b) and 7(d). In the eddy current testing probe of the selfinduction type, a single coil functions as both of an excitation coil and a detection coil, where the excitation coil generates an eddy current, and the detection coil detects impedance. In the eddy current testing probe of the mutual-induction type, an excitation coil (primary coil) and a detection coil (secondary coil) are provided separately.
As described above, the eddy current testing probes can be sorted into the four types as illustrated in FIGS. 7(a) to 7(d), according to their modes of detection of the change in the impedance of the detection coil and their constructions of the excitation coils and the detection coils.
The basic constructions and operations of the eddy current testing probes illustrated in FIGS. 7(a) to 7(d) are explained below.
In the eddy current testing probe of the absolute and selfinduction type as illustrated in FIG. 7(a), an excitation and detection coil 59 which functions both of the excitation coil and the detection coil is arranged to face a specimen (a planar object to be tested) 10. An oscillator (not shown) and an instrument (not shown) for monitoring the impedance are connected to the excitation and detection coil 59. The oscillator is provided for supplying an alternating current to the excitation and detection coil 59.
In order to detect a flaw in the specimen 10 by using the eddy current testing probe having the above construction, first, an alternating current from the oscillator is supplied to the excitation and detection coil 59 for generating an alternating magnetic field as illustrated by the arrows F1 and F2, so that an eddy current is generated at the surface of the specimen 10. Thus, impedance corresponding to the eddy current is generated in the excitation and detection coil 59. If a flaw exists at the surface of the specimen 10, the eddy current changes, and thus the impedance of the excitation and detection coil 59 also changes. Therefore, the flaw in the specimen 10 can be detected by monitoring the impedance of the excitation and detection coil 59.
In the eddy current testing probe of the absolute and mutual-induction type as illustrated in FIG. 7(b), a detection coil 51 and an excitation coil 52 are arranged so that the detection coil 51 and the excitation coil 52 face the specimen 10, and are adjacent to each other. An instrument (not shown) for monitoring the impedance is connected to the detection coil 51, and an oscillator (not shown) is connected to the excitation coil 52.
In the above construction, an alternating magnetic field as illustrated by the arrows F3 and F4 is generated by the excitation coil 52 so that an eddy current is generated at the surface of the specimen 10. Then, impedance generated by the eddy current in the detection coil 51 is monitored to detect the flaw.
In the eddy current testing probe of the differential and selfinduction type as illustrated in FIG. 7(c), the excitation and detection coils 59a and 59b forming a pair are arranged at the same distance from the specimen 10 to face the specimen 10. An oscillator (not shown) is connected to each of the excitation and detection coils 59a and 59b for supplying an alternating current to the excitation and detection coils 59a, 59b. In addition, an instrument (not shown) for monitoring a difference between amounts of the impedance generated in the excitation and detection coils 59a and 59b is connected to both the excitation and detection coils 59a and 59b. 
When detecting a flaw in the specimen 10 by using the eddy current testing probe having the above construction, first, an alternating current from the oscillator is supplied to the excitation and detection coils 59a and 59b to generate an alternating magnetic field as illustrated by the arrows F5 and F6 by the excitation and detection coil 59a and an alternating magnetic field as illustrated by the arrows F7 and F8 by the excitation and detection coil 59b, so that eddy currents are generated at the surface of the specimen 10. At this time, impedance is generated in each of the excitation and detection coils 59a and 59b corresponding to the eddy current. When no flaw exists at the surface of the specimen 10, the state of the surface is uniform, and the distribution of the eddy current generated at the surface of the specimen 10 is also uniform. Therefore, amounts of the impedance generated in the respective excitation and detection coils 59a and 59b are identical. On the other hand, if a flaw exists at the surface of the specimen 10, the distribution of the eddy current generated at the surface of the specimen 10 is not uniform due to the existence of the flaw. Therefore, amounts of the impedance generated in the respective excitation and detection coils 59a and 59b become different. Thus, the flaw can be detected by monitoring a difference between the amounts of the impedance generated in the excitation and detection coils 59a and 59b. 
In the eddy current testing probe of the differential and mutual-induction type as illustrated in FIG. 7(d), an excitation coil 52 and a pair of detection coils 51a and 51b are arranged to face the specimen 10, where the pair of detection coils 51a and 51b is located nearer to the specimen 10 than the excitation coil 52. An oscillator (not shown) is connected to the excitation coil 52, and an instrument (not shown) for monitoring a difference between amounts of the impedance generated in the detection coils 51a and 51b is connected to the detection coils 51a and 51b. The detection coils 51a and 51b are arranged at the same distance from the specimen 10 so that the detection coils 51a and 51b are symmetrically located with respect to a centerline of the excitation coil 52 in a plan view seen from the top of FIG. 7(d), and are located at equivalent positions relative to the specimen 10 and the excitation coil 52. That is, the detection coils 51a and 51b are arranged under the equivalent conditions with respect to the eddy current generated in the specimen 10 by the excitation coil 52.
In the above construction, a flaw can be detected by generating an alternating magnetic field as illustrated by the arrows F9 and F10 to generate an eddy current at the surface of the specimen 10, and monitoring a difference between amounts of impedance generated in the detection coils 51a and 51b corresponding to the eddy current.
Incidentally, in order to efficiently detect a flaw, a multicoil eddy current testing probe is proposed. In the multicoil eddy current testing probe, a plurality of detection coils or excitation and detection coils as above are arranged in a row, for example, in the direction of the width of the specimen 10. FIG. 9 shows a multicoil eddy current testing probe 20a of the selfinduction type, in which a plurality (five, in the illustrated example) of excitation and detection coils 59a to 59e are arranged in a row, and FIG. 10 shows another multicoil eddy current testing probe 20b of the mutual-induction type, in which a plurality (five, in the illustrated example) of excitation coils 52a to 52e and a plurality (tens in the illustrated example) of detection coils 51a to 51j are arranged in rows.
When the above plurality of coils in the multicoil eddy current testing probe 20a or 20b are arranged corresponding to the width of the specimen 10, and the multicoil eddy current testing probe 20a or 20b is moved in the direction over the specimen 10 as illustrated by the arrows A1 in FIG. 9 or B1 in FIG. 10, a relatively wide range of the specimen can be examined at a time, even if the specimen 10 is a fIat plate of great width.
For example, coils having a shape of a bobbin or pancake are usually used as the above detection coils, excitation coils, and excitation and detection coils.
However, impedance generated in a detection coil varies with a distance between the detection coil and the specimen. Therefore, if the distance between the detection coil and the specimen varies (as lift-off), the impedance of the detection coil changes corresponding to the lift-off, even if no flaw exists. (At this time, the change in the impedance is called a lift-off signal.) Thus, there is a problem that the conventional eddy current testing probe cannot accurately detect a flaw due to the lift-off signal.
In the eddy current testing probes of the absolute type, which detect a flaw of a specimen by a single detection coil as illustrated in FIGS. 7(a) and 7(b), impedance generated in the excitation and detection coil 59 or the detection coil 51, per se, is monitored by an impedance monitor circuit. Therefore, when lift-off occurs between the eddy current testing probe and the specimen, impedance of the coil caused by the lift-off, i.e., the lift-off signal, is noise. Thus, the impedance which can be measured by the impedance monitor circuit is affected by the lift-off signal, and therefore it is impossible to accurately detect a flaw.
In addition, in the eddy current testing probes of the differential type as illustrated in FIGS. 7(c) and 7(d), a flaw of a specimen is detected based on a difference between amounts of impedance generated in two detection coils. For example, in the eddy current testing probe of the differential and mutual-induction type as illustrated in FIG. 7(d), as long as the distance l1 between the detection coil 51a and the specimen 10 and the distance 12 between the detection coil 51b and the specimen 10 are equal, amounts of impedance generated in the detection coils 51a and 51b are identically changed even when the distances l1 and l2 vary. That is, a difference between amounts of impedance generated in two detection coils is not affected by the distance l1, l2 (normally l1=l2). Therefore, when the two detection coils 51a and 51b lift off, maintaining the parallelism with the specimen, i.e., when the two detection coils 51a and 51b lift off in a parallel lift-off mode, the flaw of the specimen 10 can be accurately detected without influence of the parallel lift-off.
However, when the two detection coils 51a and 51b lift off in a tilted lift-off mode, the row of the two detection coils 51a and 51b tilts with respect to specimen 10, i.e., when the distances l1 and l2 become different, as illustrated in FIG. 8, the amounts of impedance generated in the two detection coils 51a and 51b becomes different corresponding to the distances 11 and 12. Therefore, a difference between the changes in the amounts of the impedance generated in the two detection coils 51a and 51b arises as a lift-off signal, and a flaw cannot be detected accurately.
Although the above problem is explained for the eddy current testing probe of the differential and mutual induction type as an example, the eddy current testing probe of the differential and selfinduction type has the same problem.
FIGS. 11(a) and 11(b) shows a distribution of detectivity of the excitation and detection coils 59a and 59b in the multicoil-type eddy current testing probe as illustrated in FIG. 9. FIG. 11(a) is a diagram illustrating a geometrical relationship between the excitation and detection coils 59a and 59b and a flaw 11, and FIG. 11(b) is a diagram illustrating amplitudes (signal levels) of detection signals obtained by the excitation and detection coils 59a and 59b. In FIG. 11(b), the abscissa indicates a distance L from a reference line CL0 which is located at the center of the axis centerlines of the excitation and detection coils 59a and 59b, and the ordinate indicates a signal level detected by each of the excitation and detection coils 59a and 59b when the flaw is located at the position of the abscissa. Curves Lx and Ly show distributions of detectivity by the excitation and detection coils 59a and 59b, respectively. The signal level of the excitation and detection coil 59a is maximized when the flaw 11 is located just below the excitation and detection coil 59a, i.e., the flaw 11 is located on the axis centerline CLx of the excitation and detection coil 59a. Therefore, the distribution curve Lx of detectivity of the excitation and detection coil 59a has the maximum on the axis centerline CLx. Similarly, the distribution curve Ly of detectivity of the excitation and detection coil 59b has the maximum on the axis centerline CLy.
There is a low-detection-level region (a region in which the detection level is low) between the excitation and detection coils 59a and 59b. Therefore, when a flaw 11 is located in the vicinity of the reference line CL0, the flaw 11 may not be accurately detected by the multicoil eddy current testing probe 20a or 20b as illustrated in FIGS. 9 and 10, in which a plurality of detection coils or excitation and detection coils are arranged in only one row.
The low-detection-level region can be reduced by decreasing the distances between the adjacent detection coils or excitation and detection coils. Therefore, in order to reduce the low-detection-level region (to flatten the distribution of detectivity of a flaw signal), a multicoil eddy current testing probe 20c of the two-row selfinduction type as illustrated in FIG. 12 is proposed. In the eddy current testing probe 20c, a plurality (nine, in the illustrated example) of excitation and detection coils 59f to 59n are arranged in two rows, and the positions of the excitation and detection coils 59f to 59i in the first row located on the front side are relatively shifted from the positions of the excitation and detection coils 59j to 59n in the second row located on the rear side in the direction of the width (i.e., in the direction perpendicular to the direction of movement of the multicoil eddy current testing probe 20c) so as to decrease distances between adjacent excitation and detection coils, and thus flatten the detection level distribution of the flaw signal.
Incidentally, it is effective to produce a perspective view as illustrated in FIG. 13 for evaluating the shape of a flaw intuitively and accurately. In FIG. 13, the X-axis and Y-axis indicate a two-dimensional position on the specimen 10, and the Z-axis indicates the level of a detection value detected by the multicoil eddy current testing probe at each point indicated by the X-Y coordinates. However, it is very difficult to produce the perspective view based on the detection result by the multicoil eddy current testing probe 20c. 
Since, in the eddy current testing probe 20c, the plurality of excitation and detection coils 59f to 59n are arranged in two rows in the direction perpendicular to the direction of movement of the multicoil eddy current testing probe 20c, the excitation and detection coils 59f to 59i in the first row and the excitation and detection coils 59j to 59n in the second row perform detection at different positions in the direction of movement of the multicoil eddy current testing probe 20c at the simultaneous detection. That is, there are gaps between the positions of detection. The correspondences between two-dimensional positions on the specimen 10 and the detection values at the two-dimensional positions are essential for producing a perspective view as illustrated in FIG. 13, and therefore it is necessary to correct the detection values with respect to positions, taking the above gaps into consideration. However, the moving speed of the multicoil eddy current testing probe 20c also varies. Therefore, the above gaps vary, and thus, it is very difficult to accurately correct the detection values with respect to positions.
An object of the present invention is to provide an eddy current testing probe which can detect a flaw accurately.
In order to accomplish the above-mentioned object, an eddy current testing probe according to the present invention contains an excitation coil which generates an alternating magnetic field to generate an eddy current in a specimen; and a pair of detection coils differentially connected and arranged in phase. A central portion of the pair of detection coils and a central portion of the excitation coil are arranged to be located at an identical or an almost identical position in a plan view taken in the direction toward the specimen, and a flaw on the specimen is detected based on a difference between voltages generated in the pair of detection coils due to the eddy current.
When the eddy current testing probe is constructed as above, even if distances from the respective detection coils to the specimen are different, a sum of magnetic fluxes which are generated by the eddy current, and act on the detection coils is not changed. Therefore, the flaw on the specimen can be accurately detected based on the difference between voltages generated in the pair of detection coils by the magnetic fluxes, even in the case wherein the detection coils moves in the tilted lift-off mode in which a difference arises between the distances from the detection coils to the specimen, in addition to the case of the parallel lift-off mode in which the parallelism between the detection coils and the specimen is maintained.
In addition, it is preferable that the eddy current testing probe is configured so that the excitation coil generates the eddy current in a slanting direction with respect to the direction of the flaw on the specimen.
When the eddy current testing probe is configured like this, i.e., when the eddy current testing probe is configured so that the excitation coil generates the eddy current in a slanting direction with respect to the direction of the flaw on the specimen, the eddy current is effectively influenced by the flaw on the specimen, and thus the flaw on the specimen can be detected further accurately.
Further, it is preferable that the detection coils in the pair in the eddy current testing probe are arranged on a plane, side by side, and symmetrically with respect to a line.
When the eddy current testing probe is constructed like this, i.e., when the detection coils in the pair in the eddy current testing probe are arranged on a plane, side by side, and symmetrically with respect to a line, the detection coils are under equivalent conditions concerning the eddy current, and thus the flaw on the specimen can be detected further accurately.
Furthermore, it is preferable that the eddy current testing probe contains a bridge circuit which is connected to the pair of detection coils for obtaining as a flaw signal the difference between voltages generated in the pair of detection coils due to the eddy current.
In this construction, the flaw can be automatically detected by the bridge circuit.
Otherwise, in order to accomplish the aforementioned object, an eddy current testing probe to detect a flaw on the specimen according to the present invention contains a plurality of excitation coils which generate an alternating magnetic field to generate an eddy current in a specimen; and a plurality of thin-film detection coils which are arranged in upper and lower layers in a row.
Since the above eddy current testing probe contains a plurality of thin-film detection coils which are arranged in upper and lower layers in a row, it is not necessary to correct detection values with respect to positions in the up-and-down direction and in the direction of movement of the eddy current testing probe, i.e., in the directions perpendicular to the direction of the row. That is, it is not necessary to correct detection values between the detection coils in the upper layer and the detection coils in the lower layer. Therefore, the flaw on the specimen can be detected accurately.
In the above eddy current testing probe, it is preferable that the positions of the detection coils in the upper layer are relatively shifted by about a half pitch from the positions of the detection coils in the lower layer.
When the eddy current testing probe is constructed like this, i.e., when the positions of the detection coils in the upper layer are relatively shifted by about a half pitch from the positions of the detection coils in the lower layer, it is possible to reduce the distances between adjacent ones of the plurality of thin-film detection coils, and thus flatten a distribution of detectivity of the eddy current testing probe as a whole. Therefore, the flaw on the specimen can be detected further accurately.
In addition, it is preferable that a thin-film insulation layer is inserted between the upper layer and the lower layer.
When the eddy current testing probe is constructed like this, i.e., a thin-film insulation layer is inserted between the upper layer and the lower layer, it is possible to prevent interaction between the detection coils in the upper layer and the detection coils in the lower layer. Therefore, the flaw on the specimen can be detected further accurately.
Further, it is preferable that the above insulation layer has a through hole for leading out a signal wire of each detection coil in the lower layer.
Since a signal wire of each detection coil in the lower layer can be led to the upper side through the through hole provided in the insulation layer, it is unnecessary to provide space for a signal wire on the underside of the detection coils, and therefore it is possible to put the eddy current testing probe closer to the specimen, and suppress attenuation of a flaw signal. Thus, the flaw on the specimen can be detected further accurately. In addition, since the signal wire, which is conductive, is not arranged between the detection coils in the lower layer and the specimen, accuracy of the detection by the detection coils is not degraded.
Furthermore, in the above eddy current testing probe, preferably, each of the above excitation coils may be realized by a circular coil having a small length in the direction of the axis thereof and being arranged to stand almost perpendicularly to the surface of the specimen, and the above plurality of excitation coils may be arranged in one or more rows above the plurality of thin-film detection coils.
When the eddy current testing probe is constructed like this, i.e., when each of the above excitation coils is realized by a circular coil having a small length in the direction of the axis thereof and being arranged to stand almost perpendicularly to the surface of the specimen, and the above plurality of excitation coils are arranged in one or more rows above the plurality of thin-film detection coils, spatially dense probe configuration is realized, and efficient excitation is enabled. In addition, downsizing of the eddy current testing probe can be realized, and therefore handleability of the eddy current testing probe is enhanced.
In addition, it is preferable to construct the eddy current testing probe so that each excitation coil is oriented in a slanting direction with respect to the direction of the row of the plurality of excitation coils.
When the eddy current testing probe is constructed like this, i.e., when each excitation coil is oriented in a slanting direction with respect to the direction of the row of the plurality of excitation coils, further downsizing of the eddy current testing probe can be realized, and therefore handleability of the eddy current testing probe is further enhanced.
Further, it is preferable to construct the eddy current testing probe so that voltages are not applied concurrently to adjacent ones of the plurality of excitation coils.
When the eddy current testing probe is constructed like this, i.e., when voltages are not applied concurrently to adjacent ones of the plurality of excitation coils, it is possible to prevent interaction between eddy currents which are concurrently generated at adjacent positions. Therefore, the flaw on the specimen can be detected further accurately.
Furthermore, it is preferable that the above voltages are applied to the excitation coils in a pulsed mode.
In addition, it is preferable to construct the eddy current testing probe so as to move along the surface of the specimen in the direction perpendicular to the direction of the rows of the plurality of thin-film detection coils, and detect a flaw at the surface of the specimen.
When the eddy current testing probe is constructed like this, i.e., the eddy current testing probe moves along the surface of the specimen in the direction perpendicular to the direction of the rows of the plurality of thin-film detection coils, and detects a flaw at the surface of the specimen, it is possible to detect a flaw over a wide range.